Sheila Roy is one of only 15 people worldwide to undergo the radical treatment, which involves inserting corrective genes into the brain.

Diagnosed with Parkinson's in her 40s, she has struggled with the disease for 17 years. The symptoms include severe tremors and loss of balance, making simple tasks such as writing impossible.

Doctors at Addenbrooke's Hospital in Cambridge injected a modified virus carrying the genes directly into the motor centre of her brain. The genes provide the coded instructions for proteins needed to make dopamine, a brain chemical essential for proper control of movement.

Lack of dopamine leads to the symptoms of tremor, stiffness and poor balance associated with Parkinson's.

Mrs Roy is taking part in an early-stage study of the ProSavin therapy developed by Oxford BioMedica focusing mainly on dosing and safety. Unlike conventional tablets, the therapy involves just one treatment that does not have to be repeated.